Light emitting diode (LED) is a semiconductor diode that emits incoherent monochromatic light when electrically biased in the forward direction, that LED is considered to be the next level of technology when it comes to a light bulb. LED development began with infrared and red devices made with gallium arsenide (GaAs) since 1960. Advances in materials science have made possible the production of devices with ever shorter wavelengths, producing light in a variety of colors such as LEDs made of aluminum gallium indium phosphide (AlGaInP) for emitting high-brightness orange-red, orange, yellow, and green. Recently, as the breakthrough of epitaxy technology, commercially viable blue LEDs based on the wide band gap semiconductor gallium nitride (GaN) were invented by Shuji Nakamura while working in Japan at Nichia Corporation in 1993 and became widely available in the late 1990s. They can be added to existing red and green LEDs to produce white light, though white LEDs today rarely use this principle. Most “white” LEDs in production today use a blue GaN LED covered by a yellowish phosphor coating that can be employed as the illuminating device of the future. Compared to incandescent lights, LEDs are of interest because of the inherent characteristics: (1) They are compact; (2) They have a very long life, offering high reliability; (3) They can be driven by low-voltage DC; (4) They can be modulated (turned off and on) at high speeds; (5) They have good color mixing ability, offering almost endless color changing possibilities; (6) The light produced thereby is highly directional; (7) They have good vibration resisting ability; (8) They can produce incoherent monochromatic light. In addition, LEDs are considered to be environmental friendly since there is no mercury contained therein as there are in fluorescence bulbs. However, there are still disadvantages of using LEDs that require to be overcome, which primary are as following: (a) LEDs are currently more expensive than more conventional lighting technologie, whereas the additional expense partially stems from the relatively low lumen output and drive circuitry/power supplies needed. (b) LED performance largely depends on the ambient temperature of the operating environment, whereas “Driving” an LED ‘hard’ in high ambient temperatures may result in overheating of the LED package, eventually to device failure, and thus adequate heat-sinking is required to maintain long life.
In order to enhance brightness, a high brightness LED requires a larger current, i.e. about 350 mA˜1000 mA, compared with conventional LEDs. However, the waste heat resulting from the high power operation must be effectively managed and dissipated, such that the luminescence efficiency of the LED can be enhanced while preventing the same from damaged by overheating. Currently, a LED fabrication process of flip-chip technology is commonly adopted for overcoming the problems of luminescence efficiency enhancing and heat dissipating, wherein the designing of an improved sub-mount is the focal point for improving heat dissipating ability of LEDs. There are already a variety of LED packages taking advantage of the forgoing flip-chip process and improved sub-mount.
Moreover, another LED package problem encountered is related to electrostatic discharge (ESD), that is highly probable to cause adverse affect on the quality and production yield of LED, and thus cause the production of high brightness LEDs to be more expensive. The reasoning is that LEDs can be damaged or degraded by human touch since there may be static electricity, as high as 2000V˜3000V, accumulated in a human body in dry ambient. In reality, the substrate of a red LED made of aluminum gallium indium phosphide (AlGaInP) is a semiconductor substrate, which has good conductivity and thus is capable of preventing the accumulation of static electric charges. On the other hand, blue LEDs, especially those made of InGaN, must be fabricated on a specific substrate by an epitaxy procedure, such as a sapphire substrate and a 6H—SiC substrates, for facilitating the formation of the InGaN crystal. Selecting between a sapphire substrate and a 6H—SiC substrate, the sapphire substrate is usually being adopted for fabricating a blue LED as it has characteristics superior to those of the 6H—SiC substrate. In addition to the cost of fabricating blue, green or white LEDs is much higher than LEDs of other colors, LEDs made of sapphire substrate is comparatively highly vulnerable to electrostatic discharge since sapphire is an insulation material that is easier to cause electrostatic effect comparing to red LEDs. Therefore, it is important to add certain ESD protection dies in LED packages.
ESD protection in conventional LED packages is realized by connecting a LED with a zener diode in a parallel-connection manner, that is illustrated in the circuitry shown in FIG. 1. When the circuitry of FIG. 1 is working normally while subjecting to a normal operating voltage, the high power LED 11 is subjected to a forward bias of about 3V˜4V and the zener diode 12 is subjected to a reverse bias. It is known that the breakdown voltage of a typical zener diode is about 7V, which is controlled by the doping density of the zener diode. Therefore, as the LED 11 is conducted to illuminate while subjecting to the normal operating voltage, the zener diode 12 is not conducted and waste no power. But, when a transient static of high-voltage is generated, which is ranged between 2 kV to 15 kV, both the LED 11 and the zener diode will be conducted, moreover, as the voltage of the static is exceeding the breakdown voltage of the zener diode 12, the resistance of the zener diode 12 will be far lower than the internal resistance of the LED 11 such that almost all the current resulting from the static will flow pass the zener diode 12, and thus the operating voltage is stabilized and the LED 11 is protected from the ESD.
There are already a variety of LED packages using zener diode for ESD protection. One such LED package is disclosed in U.S. Pat. No. 6,054,716, entitled “Semiconductor light emitting device having a protecting device”, shown in FIG. 2. As seen in FIG. 2, a LED 53 is disposed at the bottom of the bowl-like structure 61 arranged on top of the leadframe 52a while a zener diode 55 is disposed at the top of the bowl-like structure 61. Since the LED 53 and the zener diode 55 are all connected by a wire bonding method, there are three bonding wires 66, 67, 68 and two pads 63, 65 arranged over the top of the LED 53, which are going to block the light emitting from the LED 53 and thus cause the brightness of the LED package to reduce greatly. In addition, as the LED 53 is connected to the bottom of the bowl-like structure 61 through its sapphire substrate 57, whose thermal conductivity is comparatively pretty low, i.e. about 25 W/m*K, such that the waste heat generated by the LED 53 can not be discharge smoothly. Therefore, the LED package shown in FIG. 2 is not suitable to be applied in high brightness applications.
Another such LED package is disclosed in U.S. Pat. No. 6,333,522, entitled “Light-emitting element, semiconductor light-emitting device and manufacturing methods thereof”, shown in FIG. 3. As seen in FIG. 3, a LED 1 is mounted face-down on a zener diode 2′ by a flip-chip process, wherein the LED 1 is parallel-connected to the zener diode 2′ by connecting the p-side electrode 5′ of the LED 1 to the n-side electrode 8 of the zener diode 2′ while connecting the n-side electrode 6′ of the LED 1 to the p-side electrode 7′ of the zener diode 2′ and a p-type semiconductor region 21 of the zener diode 2′ is formed by selectively implanting impurity ions into an n-type silicon substrate 20. As shown in FIG. 3, The LED package is characterized in that a the LED 1 is mounted on leadframes 13a and 13b with the zener diode 2′ having p-side and n-side electrodes interposed therebetween, not directly on the leadframes, wherein the zener diode 2′ is die-bonded to the die pad with an Ag paste 14, while having an n-side electrode 9 on the back face thereof in contact with the die pad of the leadframe 13a; and the p-side and n-side electrodes 5′ and 6′ of the LED 1 are electrically connected to the n-side and p-side electrodes 8 and 7′ of the zener diode 2′ via Au microbump 12′ and 11′, respectively, while the p-side electrode of the zener diode 2′ is connected by wire bonding to the lead frame 13b via an Au wire 17. Since the connection of the LED package of FIG. 3 is realized by a flip chip process, there is no bonding wire crossing over the light emitting surface of the LED 1 such that the obstruction of light caused by the disposition of wires and bonding pads, as those shown in FIG. 2, can be prevented. However, the LED package of FIG. 3 still has shortcomings listed as following:                (1) Since the alignment of the LED 1 and the zener diode 2′ is difficult to realize, the production yield is difficult to increase.        (2) In order to prevent the overflow of the Ag paste 14, the thickness of the zener diode 2′ must exceed a specific thickness.        (3) The size of the Au microbumps 11′, 12′ must be small enough since over-sized Au microbumps 11′, 12′ might cause shortage between the n-side and p-side electrodes 8 and 7′ of the zener diode 2′.        (4) Since the sub-mount of the LED 1 is a silicon-based zener diode 2′ that its thickness is specified to exceed a certain limit, the heat dissipating ability of the LED package is not satisfactory comparing to those made of metal substrate with low thermal resistance.        
Please refer to FIG. 4 and FIG. 5, which are respectively a schematic diagram showing a LED package of Lumileds Lighting, LLC., and a schematic diagram showing a circuit of the LED package of FIG. 4. The LED 7″ shown in FIG. 4 claims to be the brightest point light source currently available, that is fabricated by a principle similar to that of FIG. 3 as the sub-mounts of the two LEDs are all made of silicon, while enabling zener diodes 72 to be formed in the silicon sub-mount 71. The difference between the two LED packages is that a pair of zener diodes 72, arranged back to back, are formed in the sub-mount 71 for ESD protection, instead of only one zener diode being formed. In addition, instead of the leadframes of FIG. 3, the bottom of the LED 7″ is connect to a flat metal block 73 of high conductivity, such as copper or aluminum, that is employed as heat dissipating path and thus the overall thermal resistance of the LED package is reduce. Nevertheless, since the forgoing LED package still use silicon sub-mount, its thermal resistance, similar to that of FIG. 3, is still not satisfactory.
From the prior-art LED packages described above, one can conclude that current high power LEDs with ESD protection ability still have shortcomings as following:                (1) If the electrical connection of a LED is enabled using a wire bonding means, the heat generated thereby must be dissipated through its sapphire substrate of low thermal conductivity, such that the overall thermal resistance of the LED package can not be reduced.        (2) If the bonding wires and bonding pads are disposed at the discharging direction of the light emitted by the LED, not only the light emitted thereby is blocked, but also the light emitting area is decreased, and thus the brightness of the LED is reduced.        (3) Since the thermal resistance of conventional leadframe-type LED package is comparatively too large, a larger, thinner sub-mount is required for reducing thermal resistance of the overall LED package.        (4) If zener diodes are adopted as sub-mount of an LED package, the thickness thereof is restricted in order to prevent the overflow of silver paste such that thickness of the LED package can not be reduced.        (5) Although the heat dissipating efficiency of a silicon sub-mount is acceptable it is still inferior to that of metal substrate.        
Therefore, an improved light emitting device assembly with ESD protection ability is required.